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Researchers have examined whether certain physical characteristics are associated with sexual orientation to gain insight into the mechanisms that may be implicated in its development. Three relatively new and/or understudied physical correlates (height, birth weight, facial structure) were investigated to determine whether they are reliably associated with sexual orientation and to gain insight into the specific mechanism(s) that may be driving the association between these physical correlates and sexual orientation. In Study 1, gay men were found to be shorter, on average, than heterosexual men in a nationally representative US sample. There was no significant height difference between lesbian and heterosexual women. No evidence was found that stress and nutrition at puberty mediated the association between sexual orientation and height in men. Thus, other mechanisms (e.g., prenatal hormones, genetics) likely explain the sexual orientation-height link. In Study 2, firstborn gay male only-children had, on average, a significantly lower mean birth weight than firstborn children in four other sibship groups. There was also evidence of increased fetal loss among mothers of gay male only-children. Birth weight and fetal loss have been shown to be indicators of a mother’s immune system responding to a pregnancy. Thus, Study 2 provides support for the idea that a maternal immune response (and one that appears to be distinct from the maternal immune response hypothesized to explain the traditional fraternal birth order effect) is implicated in sexual orientation development. In Study 3, lesbian and heterosexual women differed in 17 facial features (out of 63) at the univariate level, and four were unique multivariate predictors. Gay and heterosexual men differed in 11 facial features at the univariate level, and three were unique multivariate predictors. Some of the facial features related to sexual orientation implicated a sexual differentiation related mechanism (e.g., prenatal hormones), whereas others implicated a non-sexual differentiation mechanism (e.g., developmental instability) to explain the sexual orientation-facial structure association. In addition to extending the empirical literature on the physical correlates associated with sexual orientation, the studies included in this dissertation extend our understanding of the various mechanisms likely implicated in the development of sexual orientation.